High tenacity and high modulus polyethylene fibers, due to their superior chemical and mechanical properties, such as low density, excellent chemical, ultraviolet ray and abrasion resistance, and excellent impact strength, have been widely used in the manufacturing a wide variety of industrial articles, and are considered an important engineering plastics.
A number of the processes for producing high tenacity and high modulus polyethylene fibers have been disclosed in the prior art. These include the ultra-high drawing method, solid state extrusion method, zone drawing method, and gel spinning method. Among these methods, only the gel spinning method has proved to be feasible for large scale productions and has been commercialized. The reason that the gel spinning method is particularly suitable for manufacturing high tenacity and high modulus polyethylene fibers is that the dissolution of polyethylene by solvent to form a very dilute solution enables a reduction of the number and degree of entanglements among the various polyethylene molecular chains, thus facilitating subsequent stretching of the polyethylene molecules and allowing the same to more easily assume a fully extended chain conformation. By assuming the fully extended chain conformation, the polyethylene molecules can ultimately be spun into high tenacity and high modulus fibers.
A number of different gel spinning methods have been commercialized. For example, Spectra (which is a trade name for polyethylene fibers made by Allied Corporation, U.S.A.), Dyneema (a trade name for polyethylene fibers made by DSM Corporation, The Netherlands, and Toyobo K.K., Japan), and Tekmilon (a trade name for polyethylene fibers made by Mitsui Petrochemical Industries, Ltd. Japan) are all commercially available polyethylene fibers manufactured by the gel spinning method, and all have a tenacity of at least 30 g/denier.
The gel spinning methods for manufacturing the polyethylene fibers in the above-mentioned commercialized products all comprise the steps of forming a solution of ultra high molecular weight polyethylene in a nonvolatile solvent, extruding the polyethylene solution through a plurality of capillary holes to form gel fibers therefrom, extracting the gel fibers with a volatile solvent, winding up the extracted fibers, and finally stretching the wound-up fibers to produce the final product.
The most commonly used solvents for the preparation of gel solution of polyethylene are decalin, paraffin oil, dodecane, xylene, toluene, trichlorobenzene, and tetralin. For example, Allied Corporation and Mitsui Petrochemical Industries, Ltd. use paraffin oil as the solvent for dissolving polyethylene, while DSM Corporation and Toyobo K.K. use decalin as the solvent for dissolving polyethylene. However, the prior art solvents have a number of disadvantages, for example, in that they either do not have a sufficient solubility, or the extraction rate of these solvents from the gel fiber is generally unsatisfactorily slow, or both. As a consequence of the slow extraction rate, The gel fibers must be wound up after the extraction step, but prior to the stretching step, in order to accommodate the slow extraction rate which requires a relatively low spinning speed. In other words, the slow extraction rate of the prior art process necessitates an intermediate winding step between the extraction step and the stretching to allow for a slower spinning rate. These disadvantages of the prior art methods thus necessitate the division of the polyethylene making process into three separate stages: spinning, extraction and stretching. This slows down the spinning rate and burdens the production efficiency of the gel spinning process in making polyethylene fibers.